The present invention applies to materials, or to mixtures of materials, which may be formed into a consolidated solid by application of pressure which causes the materials to be compacted. Of particular interest are materials known to form very high performance low thermal conductivity products which are known as microporous thermal insulation materials.
Blocks of microporous thermal insulation may be formed by intimately mixing a finely divided silica powder with an infra-red opacifier and reinforcing fibre and then applying pressure to the mixture to compact and consolidate the materials and in so doing to create between adjacent silica particles a bond of sufficient strength to provide a handleable block of material. Simple shapes such as rectangular slabs may readily be formed in this manner and slabs with a non-uniform thickness may be formed so that the final product has a profile. However, with profiled shapes problems can occur because of the nature of the insulation material. Although the flow characteristics of the unconsolidated insulation material are similar to a fluid, the material quickly loses its ability to flow with even a small amount of compaction so that a profiled shape has a non-uniform density.
When the profile is such that some portions of the slab have a thickness of example one half the thickness of other portions of the slab there may be a difference in density between the portions of different thickness such that the density of the relatively thin portions is approximately double the density of the relatively thick portions. This may mean that the relatively thick portions are soft, whereas the relatively thin portions are particularly hard and this can give rise to difficulties in handling the final product. Cracking of the profiled slab at points where there is a change in thickness is common.
For some insulation shapes such differences in density are acceptable, but frequently this is not the case and uniformity of density is highly desirable.
An important case where uniformity of density, and therefore strength, is essential is when the shape is a half pipe section. Non-uniformity of density generally causes cracks to occur and results in areas of weakness.